iOS/Mac/Web: Syncing and minimalist to-do lists certainly aren't hard to come by, but the iPhone, Mac, and web app Silo also adds the ability to work collaboratively on lists to mix.

Silo manages to combine the best features of a simple to-do list like Clear, with something like the previously mentionedOrchestra that offers simple sharing of tasks and lists. Silo is simple, and very clean, but manages to have enough functionality that it's still useful. The iPhone, Mac, and web app are also always in sync, so when you're sharing and editing lists as a group it works well no matter what platform everyone is on.